Known position-measuring devices typically include one or more material measures as well as one or more scanning units movable relative thereto. The material measures generally provided are linear scales or grid scales having corresponding gratings thereon. The sought position of the scanning unit relative to the material measure is determined by scanning the grating by means of the scanning unit. Known for this purpose are, addition to optical scanning principles, magnetic, inductive and capacitive scanning principles. The accuracy with which this position can be measured depends, inter alia, to a large extent on the accuracy of the grating on the material measure. In the case of optical scanning, such gratings can be produced with sufficient accuracy for normal measurement applications, so that no further corrections are necessary. However, in high-precision measurement applications, such as, for example, in lithography machines for wafer exposure, the position must be sensed with extreme accuracy, which typically requires further corrections.
In this connection, it is known to create correction tables for the individual material measures used, either during manufacture thereof or during special calibration procedures. Such correction tables specify the deviation between the position sensed by the scanning unit through scanning of the material measure and the physical position. In this regard, reference may be made, for example, to US 2008/105026 A1. In a first step during the actual measurement operation, an uncorrected position value is then determined by scanning the material measure by means of one or more scanning units. In another, additional correction step, a correction value from the correction table is combined with the measured uncorrected position value, for example added thereto or otherwise arithmetically combined therewith, in order to generate a corrected position value.
In such high-precision measurement applications, the position values generated by the position-measuring device are typically further processed in real time, for example, in a downstream control system for positioning a stage in a lithography machine. It is therefore required that the corrected position value also be generated in real time and with a minimum of additional processing time. Typically, in highly dynamic applications, only a few microseconds are available for this purpose.
Highly accurate position-measuring devices typically require positional resolutions of a few 10 picometers in such applications; i.e., the correction table used must also have this resolution. To provide a sufficient degree of accuracy, the correction values must be provided in the correction table with a correction pitch of about 0.1 mm-1 mm. In applications, where two-dimensional scale plates are used as material measures, the correction values must also be available in two-dimensional form for the entire measurement range. As a result, on the one hand, correction tables containing several millions of individual correction values are typically required and, on the other hand, 16 or more bits are needed for the binary representation of an individual correction value because of the required high positional resolution in the picometer range. Thus, the amount of memory required increases substantially with increasing size of such correction tables.
The position-measuring devices typically have signal-processing units associated therewith for processing the generated signals. Such signal-processing units may be placed near the scanning units, but also further away therefrom. In addition to various signal-processing elements, the signal-processing unit contains, inter alia, also the memory unit in which one or more correction tables are stored. These signal-processing units are often so-called “embedded systems,” which are optimized for rapid calculation and transmission of the corrected position values to a control system via a high-speed interface. In the applications mentioned, the time between scanning and the transmission of the corrected position value should be as short as possible, typically in the range of a few microseconds. This can only be ensured by using digital signal processors or programmable logic elements in the signal-processing unit, to which memory units for the required correction tables can be connected only to a limited extent.
In such systems, the evaluation unit is often connected to a higher-level machine controller via a further interface, such as, for example, a suitable field bus. This interface is also used, inter alia, for transmitting the correction tables into the memory unit of the respective signal-processing unit. The interface used for this purpose is usually not designed as a high-speed interface; i.e., not for high data throughput. Frequently, a plurality of signal-processing units of a plurality of position-measuring devices are connected to the higher-level machine controller via this interface and, therefore, the transmission of a plurality of extensive correction tables to the different signal-processing units can take a considerable amount of time. During this period, the memory units containing the correction tables are not available for measurement value correction; i.e., for the measurement operation.